Throughout this application, various publications are referenced in parentheses by author and year. Full citations for these references may be found at the end of the specification immediately preceding the claims. The disclosures of these publications in their entireties are hereby incorporated by reference into this application to describe more fully the art to which this invention pertains.
Depression is the most common of mental disorders and yet is often underdiagnosed and undertreated, inflicting substantial morbidity and psychosocial impairment on its sufferers. Depression is mainly characterized by sadness, flatness, loss of feeling, anhedonia (lack of pleasure), tearfulness, agitation or retardation, thoughts of guilt and worthlessness; in severe cases, suicide, hallucinations and delusions.
Depression can be mainly categorized into bipolar disorders, identifying wide swings of mood; major depressive illness, marked by severe depressive symptoms but without manic swings; and less defined milder forms of bipolar disorder and major depression that fall short of the specific diagnostic criteria, e.g. dysthymic disorder (formerly called depressive neurosis). The symptomatology and diagnostic criteria for depression are set out in the Diagnostic and Statistical Manual of Mental Disorders, 4th edition. Although many patients have single episodes of major depressive illness, the condition also can be repetitive, and this recurrent condition is frequently called unipolar depressive illness.
The key features of depressive illness are a markedly gloomy mood in which there is a loss of interest in life, and general feelings of hopelessness and worthlessness. Depressive symptoms range in severity from mild mood swings to severe delusions about self-worth, accomplishments, and the future.
The “blackness” of the presentation in the depressed patient is most often accompanied by severe motor retardation with profound sleep and appetite disturbance and suicidal ideation. Furthermore, depressive illness can also present in a highly anxious or agitated state.
The degree to which the underlying brain mechanisms in anxiety and depression differ or overlap remains unknown. The fact, however, that to some extent the same neurotransmitter systems are involved in depression and anxiety does not mean that the mechanisms are identical. However, the majority of people in an episode of either depression or anxiety also meet criteria for at least one other psychiatric disorder. But by far the strongest comorbidities in both cases are between depression and anxiety disorders. Therefore, it is now becoming common clinical practice to treat both indications with antidepressants such as selective serotonin re-uptake inhibitors.
The key clinical features of anxiety disorders relate to various combinations of psychological and physical manifestations of anxiety, not attributable to real danger and occurring either in attacks (panic disorder or PD) or as a persisting state (generalized anxiety disorder or GAD). Other neurotic features may be present (obsessional or hysterical symptoms) but do not dominate the clinical picture.
The Pathophysiology of Depression
Theories underlying the pathophysiology of depression have developed from several lines of evidence including: 1) changes in neurotransmitter monoamine levels; 2)endocrine imbalance; and 3) electrophysiological studies on sleep functions.
Evidence implicating the role of neurotransmitters in depression, in particular the monoamines serotonin, noradrenaline and dopamine, include the success of pharmacological agents in treating depressive disorders. Many of the tricylic antidepressants (TCAs), selective serotonin re-uptake inhibitors (SSRIs) and monoamine oxidase inhibitors (MAOIs) effective in the treatment of depression increase the availability of the catecholamines (noradrenaline and dopamine) and indolamines (serotonin) in the central nervous system (CNS). The clinical efficacy of these agents has given rise to the catecholamine-indolamine hypothesis of depression. This theory postulates that a certain level of amines and/or receptor sensitivity to catecholamines functions to generate a normal mood. A receptor insensitivity, a depletion of monoamines, or a decrease in their release, synthesis or storage have been postulated to lead to depression.
Current Treatments for Depression
A variety of pharmacological agents have been employed to treat depression based on the catecholamine-indolamine hypothesis of depression. Drugs used to treat depression include MAOIs, atypical antipsychotics, lithium, TCAs, and SSRIs. In addition, a number of off-label agents such as antiepileptics are used to treat depression in treatment-resistant patients.
Tricyclic antidepressants are about equal to SSRIs in effectiveness against depression, thus providing supporting evidence for the catecholamine-indolamine hypothesis of depression. However, SSRIs have largely displaced TCAs because of side effects associated with TCAs and the need to monitor EKG and plasma drug concentration. Although the SSRIs are viewed as an improvement over other antidepressants, they are not without their clinical problems. Adverse effects on sexual function, primarily anorgasmia and delayed ejaculation, have been consistently reported. Other, common side-effects include sleep disorders, yawning, weight changes, suicidal ideation and extrapyramidal-like side-effects such as dystonic reactions. Thus, there clearly remains a medical need for new treatments of depression, without the adverse side effect profile of existing agents and with improved efficacy.
Current Treatments for Anxiety
There is now considerable direct evidence for the efficacy of the SSRIs both in depression and in anxiety disorders.
Of the current SSRIs approved for marketing in the United Staes all have shown sufficient efficacy to be further approved for the treatment of at least one anxiety disorder, for example obsessive compulsive disorder (OCD) or generalized anxiety disorder (GAD). Compounds such as paroxetine and sertraline are also indicated for the treatment of panic disorder (PD).
However, it is clear from the issues raised earlier relating to the efficacy and side effect profile of SSRIs and for that matter the more widely prescribed benzodiazapines, there still exists a real medical need for novel approaches for the treatment of anxiety and depression.
Discovery of GalR3 Receptor Subtype and its Role in Depression and Anxiety
The investigations leading to the present invention were preceded by the discovery that mRNA for the GalR3 receptor is localized to areas of the rat brain associated with mood and emotion, thus supporting the expression of GalR3 in those regions. GalR3 receptor protein also localizes to areas of the rat brain associated with mood and emotion.
This discovery led to the concept that the GalR3 receptor plays a role in controlling the activity of catecholamine and indolamine neurons in the CNS. Galanin is known to hyperpolarize neurons, including monoaminergic neurons (Seutin, et al., 1989) and to have inhibitory effects on 5-HT neurons (Xu, et al., 1998), and dopamine neurons (Gopalan, et al., 1993; De Weille, et al., 1989; Jansson, et al., 1989; Nordstrom, et al., 1987; Weiss, et al., 1998). A series of in vivo behavioral experiments were carried out to evaluate the antidepressant and anxiolytic properties of a selective GalR3 receptor antagonist, which established that selective antagonists of GalR3 are useful for the treatment of depression and/or anxiety. We now report novel chemical structures that are high affinity antagonists of GalR3. By virtue of their ability to antagonize the GalR3 receptor, these compounds are useful for the treatment of depression and/or anxiety.
The Rat Forced Swim Test and the rat Social Interaction Test can be employed to evaluate the use of selective GalR3 receptor antagonists to treat depression and anxiety. These models are considered by experts in the field to be predictive of the potential utility of agents to treat depression and anxiety.
Rat Forced Swim Test (FST)
The rat Forced Swim Test (FST) is a behavioral test that is used to screen compounds for antidepressant efficacy (Porsolt et al., 1977, 1978; Porsolt, 1981) This test is widely used, relatively easy to perform, and sensitive to the effects of some of the major classes of antidepressant drugs, including TCAs and MAOIs, and various atypical antidepressants. Furthermore, this test is relatively selective for antidepressant drugs in the sense that as few psychoactive drugs produce similar behavioral actions in the FST.
In the rat FST, animals are placed in a cylinder of water, from which there is no escape, for an extended period of time. Typically, animals will display a range of behaviors such as immobility, climbing, swimming, and diving, with immobility being predominant after several minutes of immersion in the water. Consequently, many past studies have only measured or scored immobility after the administration of the test agent. Unfortunately, this method does not score any other active behaviors that may be produced by potential antidepressants. Thus, if a particular class of antidepressant were to have very little effect on immobility, yet produce characteristic behaviors during the FST, these behaviors would not be scored and the conclusion would be that the compound in question does not possess antidepressant action.
Recently, however, a sampling technique was developed to score active behaviors in the FST, such as swimming, climbing and diving, in addition to immobility (Detke, et al., 1995; Lucki, 1997; Page, et al., 1999; Reneric and Lucki, 1998). This modified sampling technique has indicated that SSRIs, such as fluoxetine, paroxetine and sertraline, significantly decrease immobility and increase swimming time (Detke, et al., 1995; Page, et al., 1999). In contrast, selective reuptake inhibitors of norepinephrine (NE) increase climbing behavior but do not alter swimming time (Detke, et al., 1995; Page, et al., 1999).
Rat Social Interaction Test (SIT)
There are a number of paradigms that have been used to determine whether a compound possesses anxiolytic action. A number of these tests involve food or water deprivation, punishment or measurement of consummatory behavior (see File, et al., 1980; File, 1985; Rodgers, et al., 1997; and Treit, 1985, for review). In addition, in these models, prior conditioning reduces the uncertainty or anxiety. In general, these tests lack ethological validity.
One model that is based upon an unconditioned response that does not involve punishment or deprivation is the Social Interaction Test (SIT) (File and Hyde, 1978, 1979). In this model, rats previously housed singly are placed in a familiar, dimly lit, test arena with weight-matched, novel partners. The principal anxiogenic stimulus under these conditions is the partner novelty, which involves an unconditioned response to a potential threat. After pharmacological treatments, the following behaviors are scored as active social interaction: grooming, sniffing, biting, boxing, wrestling, following, crawling over and crawling under. A wide range of psychoactive drugs have been examined in this paradigm and it has been shown that the social interaction test can distinguish anxiolytics from antidepressants, antipsychotics, analeptics and sedative agents (File, 1985; Guy and Gardner, 1985). This test can detect anxiolytic agents such as the benzodiazepines (File and Hyde, 1978; File and Hyde, 1979; File, 1980), in addition to non-benzodiazepines, including paroxetine and other SSRIs (Lightowler, et al., 1994). Finally, the social interaction test can detect anxiogenic agents, including the inverse benzodiazepine receptor agonists (File, et al., 1982; File and Pellow, 1983; File and Pellow, 1984; File, 1985).
We now have unexpectedly discovered that the novel tricyclic imines described herein are high affinity antagonists of GalR3, and are useful, by virtue of their ability to antagonize the GalR3 receptor, for the treatment of depression and/or anxiety.
In one embodiment of the present invention the synthesis of tricyclic imines which bind with high affinity to the cloned human GalR3 receptor is disclosed. Thus, the GalR3 receptor antagonists described herein, which may be further classified as neutral antagonists, inverse agonists or allosteric modulators, provide a novel method to treat depressive disorders and/or anxiety.